The present invention relates to an electron gun printer. It is applicable to the recording of drawings or texts on a printing medium. It can be used as a computer peripheral or in drafting, drawing or text processing machines.
At present there are numerous printers operating according to different principles:
mechanical percussion printers operating on the typewriter principle;
thermographic printers, in which a sheet of special thermographic paper passes in front of static heating printing heads;
photoelectric printers using the operating principle of photocopiers;
spark-operated printers, where sparks destroy a metal coating deposited on the paper;
magnetic ink printers, in which an inking roller is locally magnetized by a series of fixed heads arranged parallel to a generatrix of the roller, whereby the ink magnetized in this way on the roller in accordance with the drawings or text to be obtained is then deposited on a sheet of paper; laser printers, in which a laser beam of modulated intensity is deflected along an alternating scanning line in front of a photoconductive roller, onto which are attracted ink particles which are then transferred to a paper sheet passing in a direction perpendicular to the scanning line;
photographic printers, in which a sheet of paper coated with a photosensitive layer passes in front of a monodimensional cathode ray tube, in a direction perpendicular to the scanning line of the spot emitted by the electron gun of the tube.
All the printers operating according to known principles suffer from numerous disadvantages, i.e. they require the use of a specific printing medium, their structure and operation are complicated, particularly for mechanical printers, while their cost is generally high.
In order to be able to use printing media of different types and simplify the structure of the printers described hereinbefore, it is also known to e.g. produce a printer with the aid of a cathode ray tube incorporating an electron gun, means making it possible to scan the electron beam in a plane along a rectilinear window closed by a plate which is transparent to the electrons. This plate is shaped like a channel returning towards the inside of the tube. This tube makes it possible to print on different media and can in particular be used in facsimile transmission machines, in which, apart from the line scan or horizontal sweep performed by the electronic beam, there is a frame or raster scan or vertical sweep of the printing medium. For example, such a printer is described in the journal EDN, vol. 14, No. 2, Jan. 15, 1969 (DENVER, U.S.A.). However, this type of printer suffers from the disadvantage of having a rectilinear window of considerable length. This window is in fact closed by a plate, which must have a very limited thickness in order that it be transparent to the electrons, while being very strong, because an ultrahigh vacuum is produced within the cathode ray tube. It is therefore difficult to find a structure which is very rigid and onto which the window can be fixed and which must necessarily stay thin. Thus, the dimensional variations of this structure under the effect of temperature or atmospheric pressure are transmitted to the window and may lead to the destruction thereof.